1. Field of the Invention
This invention relates to an apparatus for linearizing and improving the characteristics of electronic power amplifiers with regard to varying resistive and/or reactive, possibly nonlinear loads, to yield more economical amplifier designs. To this end, the apparatus incorporates the combination of semiconductor devices, current and voltage sources, and passive electronic components. In addition to these, the practical realizations of the invention may make additional use of various mechanical and electromechanical parts and components, as is customary in power amplifier circuits.
2. Description of the Prior Art
An ordinary prior-art amplifier circuit is illustrated in FIG. 1, wherein semiconductor transistors Q.sub.1A and Q.sub.1B, connected to supply voltage sources +V and -V respectively, are used as output devices for the amplifier, which feed a load L. The section driving the output devices consists of semiconductor transistors Q.sub.2A and Q.sub.2B forming a grounded collector circuit configuration, and of semiconductor transistors Q.sub.3A and Q.sub.3B which drive the transistors Q.sub.2A and Q.sub.2B in a grounded emitter circuit configuration. Diodes D.sub.1 to D.sub.4 form the necessary bias voltage in order to allow current to flow through the transistors Q.sub.2A, Q.sub.2B, Q.sub.3A, Q.sub.3B, in the quiescent state. Emitter resistors R.sub.1 and R.sub.2 are used to linearize the output section, and to prevent thermal runaway should the output devices for any reason become overheated. For reasons of linearity and speed of operation, a resistor R.sub.3 is used to pass appreciable additional current through the transistors Q.sub.2A and Q.sub.2B in excess of the base currents of the output devices.
It is to be understood that many practical variations exist of the circuit described above; and that the circuit may be realized with various kinds of semiconductor devices and may be used with asymmetric as well as with symmetric power supply sources; and that in an asymmetric amplifier version, the semiconductor devices denoted by subscript A and B respectively, may be replaced with current sources or equivalent electronic means, without departing from the basic concept of operation outlined above, and without curing any or all of the drawbacks described below.
The prior-art circuit arrangement described above has, among others, the following drawbacks: (i) The current gains .beta..sub.1 and .beta..sub.2 of the transistors Q.sub.1AB and Q.sub.2AB are dependent on the actual load current. Therefore, even in the event of a perfectly linear resistive load L, the loading seen at the collectors of the transistors Q.sub.3A and Q.sub.3B becomes nonlinear and causes appreciable distortion, i.e., non-linearity of the output voltage v.s. the input voltage. (ii) It is inherent in basic semiconductor device physics that the current gains .beta..sub.1 and .beta..sub.2 decrease rapidly after a certain current through the respective transistor is reached. This places a practical limit to the maximum output current of the circuit, which cannot be exceeded without gross nonlinearity. (iii) The base-to-emitter voltage drops V.sub.be of the semiconductor transistors Q.sub.1AB and Q.sub.2AB depend on current and temperature, thereby adding another source of nonlinearity to the circuit. This non-linearity is further aggravated by the continuous and momentary variation of the temperature of the semiconductor devices with their power dissipation, i.e., change in the product of collector current and collector-emitter voltage with the signal to be amplified. (iv) Because of the grounded collector circuit configuration of the driver transistors Q.sub.2A and Q.sub.2B can easily drive Q.sub.1A and Q.sub.1B on, i.e., to carry more current, but can only limitedly turn them off, i.e., draw away current carriers from their bases.
All these inherent drawbacks necessitate use of heavy overall feedback to correct for these degradations. Feedback, in turn, has been shown to create several other problems so that designs relying heavily on the usage of feedback must be considered inferior to designs which do not necessitate its usage.
Accordingly, it is an object of the present invention to provide an apparatus for improving the characteristics of transistorized power amplifiers, the apparatus being designed to yield a highly linear and economical high-speed amplifier circuit.